New Corn Germplasm Lines Fend Off Two Key Nematode Pests

MISSISSIPPI STATE, Miss., Sept. 16--Disaster awaits nematodes that try
to feed and house their eggs in the roots of new lines of resistant corn, U.S.
Department of Agriculture researchers said.

Now available to plant breeders, the resistant plants cause worms
hatched from eggs deposited in their roots to die quickly or take longer to
mature, said plant pathologist Gary Windham of USDA's Agricultural Research
Service. That means fewer surviving adults to continue harming the plants, he
noted.

The crop-damaging worms are the southern root-knot nematode, Meloidogyne
incognita, and the peanut root-knot nematode, M. arenaria. Left unchecked, the
pests can cause yield losses in corn of 30 percent.

ARS plant geneticist W. Paul Williams said commercial corn bred from the
resistant lines could benefit farmers in Mississippi, Alabama, Florida and other
southern states where the nematodes cause their mischief in corn and other
crops, including cotton and soybean.

Corn plants that hold their own against these pests would require little
or no treatment with chemical nematicides, according to Williams, research
leader of ARS' Corn Host Plant Resistance Research Unit, Mississippi State,
Miss.

Williams and Windham worked with Mississippi State University personnel
to develop, test and produce seed of the new corn germplasm lines, dubbed
"Mississippi (Mp) 709, 710, 711 and 712."

Williams said the yield isn't as high as commercial corn, but plant
breeders can remedy this by crossing the lines with higher-yielding varieties.

In Mississippi and other states, farmers often rotate corn with cotton
or soybeans for greater flexibility with weed-killing herbicides and less risk
of disease. But planting corn in worm-infested soils can send the pest's
numbers soaring, endangering crops planted after corn the following season,
Windham noted.

"We figure that if you plant corn that can resist the nematodes, you can
really do something to reduce their population size in a crop rotation with
cotton, for example," said Williams.

Windham noted though M. incognita is a greater threat to corn than M.
arenaria, both can be found in the same crop fields, especially those with sandy
soils. There, the tiny worms pierce the roots of corn plants to feed, mate and
later deposit hundreds of eggs each. Their feeding damage causes galling, or
knots, in the roots so the plants get fewer nutrients and less water from soil,
Windham said. This can stunt their growth.

In the greenhouse studies, the scientists applied 3,000 nematode eggs to
seedlings of each of the resistant corn lines and susceptible check lines.

Worms that hatched from the eggs and survived were allowed 60 days to
feed and mate. The researchers then counted the number of eggs deposited by the
surviving females.

Then they rated the plant's resistance or susceptibility. Of the
resistant lines, Mp 709 fared best with only an occasional egg mass (or cluster)
on the roots of each plant. Mp 710 was second best, with only one or two egg
clusters per plant. A susceptible check, Ab24E, had 65 egg masses per plant.

The two resistant lines are selections of plants from two 50-year-old
corn varieties called Old Racoon and Tebeau.

What makes them resistant to the worms is still something of a mystery,
Williams said. One possible clue may be a natural protein the plants produce
upon attack, but that suspicion has yet to be confirmed, he cautioned. "Now
that we have the resistance, we can start looking at what the mechanisms are
behind it," he said. "We are working with a Mississippi State University
molecular biologist, Dr. Nancy Reichert, to solve the mystery."

William's lab also is investigating their new corn germplasm lines for
resistance to the aflatoxin fungus Aspergillus flavus. It produces a carcinogen
in the kernels of corn and in other crops like peanut that is harmful to humans
and livestock.

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